A clear majority of senators now oppose the United Nations Arms Trade Treaty.

Because two-thirds of senators would be required to consent to ratification, the fate of the treaty was always in doubt. Now it’s unclear whether the Senate could even proceed to debate the measure.

There’s no way to filibuster a motion to take up a treaty, but a roll call vote may be required to go into executive session to consider it. That would require a simple majority, and it isn’t at all clear the treaty — which would establish new international standards related to trade in conventional weapons — would get over that threshold.

In a Monday letter to President Barack Obama, Democratic Sens. Jon Tester and Max Baucus of Montana joined Heidi Heitkamp of North Dakota and Joe Donnelly of Indiana in announcing opposition to the treaty that Secretary of State John Kerry signed on to on behalf of the Obama administration last month.

The four Democrats say that including small firearms and ammunition in the terms of the agreement “makes the Treaty overly broad.”

“We take our constitutional obligation as U.S. Senators to provide advice and consent seriously. And although we appreciate the Administration’s work to make this Treaty more amenable to the United States’ interests, we believe it falls short,” the senators wrote. “Because of unaddressed concerns that this Treaty’s obligations could undermine our nation’s sovereignty and the Second Amendment rights of law-abiding Americans, we would oppose the Treaty if it were to come before the U.S. Senate.”

The National Rifle Association, which has long opposed the treaty, praised the senators’ letter.

“This letter sends a clear message to President Obama and Secretary Kerry that the Arms Trade Treaty will not receive the 67 votes in the U.S. Senate necessary for ratification,” NRA Executive Director Chris W. Cox said in a Tuesday statement.

The letter from the four Democrats followed a letter sent last week by 50 senators, including 5 Democrats, expressing opposition to the treaty.

“The treaty includes only a weak non-binding reference to the lawful ownership and use of, and trade in firearms, and recognizes none of these activities, much less individual self-defense, as fundamental individual rights,” wrote the group of 50, led by Kansas Republican Jerry Moran, West Virginia Democrat Joe Manchin III and Oklahoma Republican James M. Inhofe.

The 50 signatories on that letter include Sen. Jeff Chiesa, R-N.J. The appointed senator’s term will soon expire with the seating of Cory Booker.

Booker has a record of supporting gun control efforts, which means the actual vote count in opposition to the treaty may soon be 53.